1. Field of the Invention
This invention relates to a process for the manufacture of chromium (III) compounds from chromate and/or dichromate salts. More particularly, this invention relates to a process for manufacturing such chromium (III) compounds which are essentially free of cations of the chromate and/or dichromate salts.
2. Description of the Prior Art
In recent years, the expanding use of chromium (III) compounds have made these materials of increasing importance. For example, chromium chloride has found utility as a precursor in the manufacture of catalysts and organic compounds of chromium. Similarly, methods have been developed whereby chromium metal may be produced from high purity chromic chloride. The development of NASA's Redox Energy System based on the use of chromic chloride has spurred interest for new inexpensive methods of preparing this compound.
A number of methods for producing chromium (III) compounds, as for example, chromic chloride have been proposed. However, these compounds, and particularly chromic chloride have remained materials which in the past have been prepared with considerable difficulty and at great expense, even on a laboratory or limited pilot plant scale. For example, in one such method for preparing chromic chloride, this material is prepared by passing chlorine gas over ferrochrome or chromium metal at high temperature. However, this process suffers from several defects the most significant of which is that it is very inefficient which results in high expenditures of time, money and man-power.
Other high cost, inefficient processes for preparing chromic chloride involve the use of chromic oxide or chromic acid as the starting materials. In one such process described in U.S. Pat. No. 3,309,172, chromic chloride is prepared by reacting chromic oxide, carbon monoxide and chlorine gas in the presence of an inert particulate material at elevated temperatures. In another such method described in U.S. Pat. No. 3,305,303, chromic chloride is prepared by reacting chromic oxide with chlorine and carbon. It is disclosed in the literature that chromic chloride may also be prepared by either reducing chromic acid with methanol in the presence of hydrochloric acid or by reacting chromic oxide with hydrochloric acid. These prior art processes which involve the use of chromic oxide or chromic acid as the starting materials suffer from a number of inherent defects, chief of which is that the cost of producing the desired product is so great that only minor amounts of the desired product can be used economically and then only in a narrow class of specific situations. The increased cost results from the need to convert chrome ore into the required chromic acid and/or chromic oxide precursors. The production of chromic acid and chromic oxide from chrome ore involves a roasting and leaching operation to convert the ore into sodium chromate, i.e. yellow liquor; acidification of sodium chromate with sulfuric acid to provide a sodium bichromate solution, followed by crystallization and drying of the resultant solid to provide anhydrous sodium bichromate; and reduction of sodium bichromate with carbon or sulfur to provide chromic oxide or combination of sodium bichromate with sulfuric acid to yield chromic acid.
It is apparent that the more desired and economical, both time, equipment and cost-wise route to chromium (III) compounds would be directly from the chrome ore, or directly from sodium chromate or sodium dichromate thereby avoiding the expensive, time consumming chrome ore conversion steps. However, heretofore, none of these prior art processes have been used to prepare chromic chloride either because of an inability to separate the desired chromium (III) compounds from impurities, primarily sodium cations, or because of the prohibitorily high time, cost, manpower and equipment expenditures required for such separation.